(a) Field of the Invention
The present invention relates to a zoom lens system and, more particularly, to a zoom lens system with the focal length of 75 mm to 150 mm, large aperture ratio (F/3.5), high performance, small number of lenses constituting the lens system, and short overall length.
(b) Description of the Prior Art
To make the aperture ratio of the above-mentioned kind of zoom lens system large to about F/3.5 and to make the overall length of the lens system short, it is known that it is necessary to sacrifice the quality of image or to make thicknesses of respective lens elements constituting the varifocal system in the marginal portion and those on the optical axis as thin as possible in order to ensure sufficient spaces for moving the varifocal lens groups.
To manufacture such lenses that thicknesses in the marginal portion and those on the optical axis are thin, it is difficult to adopt high-speed grinding and polishing enabling to reduce the cost of production and, in case of a cemented lens component, strain occurs in the lens component. Therefore, even when a lens system with high performance is designed, the performance of the actually manufactured lens system does not become so favourable. To manufacture a lens system with performance close to that of the designed lens system, accuracy of parts should be made high. As a result, the cost of production becomes high, and this is not preferable.
This kind of known telephoto zoom lens systems are disclosed in Japanese published unexamined patent applications Nos. 131852/78 and 119110/81.
Besides, Japanese published unexamined patent application No. 126819/81 also discloses this kind of zoom lens system.
However, in case of the zoom lens system disclosed in Japanese published unexamined patent application No. 126819/81, the aperture ratio is F/4.5 and this is smaller compared with the lens system according to the present invention.
Japanese published unexamined patent application No. 150717/81 also discloses this kind of zoom lens system. However, in case of said zoom lens system, the type of the afocal system is different from that of the zoom lens system according to the present invention and the lens configuration of the lens system is also different. The zoom lens system disclosed in said Japanese published unexamined patent application has such lens configuration that it is rather difficult to correct spherical aberration and astigmatiam and, especially, astigmatism thereof in the margimal portion of field is not corrected so favourably.